I'm going to write this thread and update it as I go to hopefully put an end to all the "will these rims fit/ what tire size should I get?" threads.
Wheel Sizing
The first thing you'll want to decide is what size rims you want. Our cars look great with 17s-20s depending on the style you are going for.
Width and Offset
The next thing you'll need to determine is your width and offset. These are two of the most important parameters when trying to determine if and how a wheel will fit on your car. Most 05-09 GT cars came with 17x8 or 18x8.5 (+50) from the factory and V6 cars came with 17x8 or 16x7 IIRC GT500s came with 18x9.5 (2007-2009) or 19x9.5 (2010+) or 19x9.5F 20x10R (2011 Track).
In General the baseline I use for wheel fitment is the Roush wheels which are an 18x10 +45 millimeter offset, which sit pretty flush with a 275/40/18 tire. The wider the tire, the higher the offset you will need to fit a given wheel when you are running wheels that are very close to flush with the fenders.
With this baseline you can figure out the offset you can run on a give size wheel using the offset calculator below.
In general for every addition inch of width you need to add 13mm of offset for the same fender side fitment.
So for example a 10" +45 has the same fitment on the front side as a 1 9" +32 wheel. This is a good quick rule for figuring out your wheel fitment.
The widest wheels that fit without any fender modifications are 20x11s (not sure if 19s or 18s will fit.) The correct offset for this that will allow you to run a large tire ~305 will be +59-61.
I am currently running a 19x9.5+29 on the rear of my car with a 275/40/19 and that is nearly perfectly flush with the fender. You can also use this as a baseline for aggressive fitment.
These two links will be extremely helpful in understanding wheel offset and how different wheels will fit relative to others. It's all pretty basic math.
Wheel offset explained
Wheel Offset Calculator
Common Offsets
9" - +24 to +45
9.5" - +30 to +45
10" - +36 to +51
10.5" - +42 to +57
11" - +59 or greater
Tire Sizing
Once you've decided on a wheel, you need tires, unless you plan on driving your car down the train tracks. Every tire has a recommended width range, which is the width of the rims that a given tire size will fit on. In general you can safely go a half inch wider than the widest recommended wheel, but you really should not go narrower than the recommended width because the dynamics of the tire will be severely negatively affected (for street tires).
There are a number of things to look at when selecting a tire.
Tire sizes are usually expressed in XXX/YY/ZZ
XXX - Treadwidth, width of the tire in mm
YY - Aspect Ratio, the sidewall height as a percentage of the treadwidth.
ZZ - Size of the wheel/ rim that the tire is designed for.
Ex. 255/40/19 255mm (~10.08") width tire, with a 102mm (~4.01") Sidewall on a 19" wheel.
The total tire height in inches = [2(YY)(XXX)/254] + ZZ
using example above the tire height is = [2(40)(255)/254] + 19 = 27.02
Stock tire height is ~27" so you want to keep that in mind when looking at tire sizes.
Ranges For Rim Sizes(based on 19-20" fitment/ may vary for 17-18"):
8.5" 235-255
9" 245-275
9.5" 245 -285
10" 255-295
10.5" 275-305
11" 275-315
The following links will help quite a bit when trying to figure out tire fitment. The tire stretch calculator will give you an idea of how a given wheel/tire combo will fit relative to another. The only thing to remember is that real wheels are measured from the insides of the lips and the calculator does not account for the outer lip, but it is still a good tool for comparison.
Tire Size Explained
Tire Size Calculator
Tire Stretch Calculator
Common Wheel/Tire Size Combos
17x8 (stock) - 235/55/17 and 255/50/17
18x8.5 (stock) - 235/50/18 and 255/45/18
18x9 (+30ish)- 255/45/18 and 275/40/18
18x9.5 (+44 GT500) - 255/45/18 Front 285/40/18 Rear
18x10 - 275/40/18 or 285/40/18
19x9.5 (+33 RTR) - 245/(40 or 45) {slightly stretched} 255/(35 or 40) 275/(35 or 40) or 285/35 [can be mixed for stagger]
19x10 -255/(35 or 40) {slightly stretched} 275/(35 or 40) or 285/35
19x10.5 275/(35 or 40) {slightly stretched} or 285/35
20x8.5 (foose nitrous front) - 255/35/20
20x9 (saleen fronts) - 255/35/20
20x10 (foose nitrous and saleen rears) - 275/35 or 275/30 or 285/30
Added from post below:
The spreadsheet is located http://www.lowered.eu/misc/files/Tyre_Calculator_English.xls
/\ no longer works:
TRC51 said:
NOTE TO OP:
The spreadsheet at the bottom of your original post has gone missing. I used that a lot, so I went searching and think I found a very nice replacement. This shows measurement differences and everything!
Here is a visual to show you using an .xls spreadsheet. I am showing this comparing a well known acceptable combination of 275/40/18 on 18x9.5 +45 compared to he drifts 295/35/19 19x9.5.
